To protect against this, compose() allow only keys named in 'Content-Type' way and will throw exception if it found keys named in other way. There few exceptions from this rule: 'ETag', 'WWW-Authenticate' and 'Digest-MD5'.

You can set it to usual string (like 'Sat, 01 Jan 2000 00:00:00 GMT') or to unixtime in seconds (as returned by time()) - in later case time in seconds will be automatically converted to string with date/time.

If it set to empty string (new() will initially set it this way), then current date/time will be automatically used.